1. Technical Field
This disclosure relates generally to image sensors, and in particular but not exclusively, relates to CMOS image sensors.
2. Background Art
Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as medical, automobile, and other applications. The demands of higher resolution and lower power consumption have encouraged further miniaturization and integration of these image sensors. As a result, technology used to manufacture image sensors, for example, CMOS image sensors (“CIS”), has continued to advance at a great pace.
FIG. 1 is a circuit diagram showing pixel circuitry 100 including two four-transistor (“4T”) pixel cells—Pa 110a and Pb 110b—of a conventional pixel array. In FIG. 1, pixel cells Pa 110a and Pb 110b are arranged in two rows and one column. Pa 110a and Pb 110b each include the same conventional pixel cell architecture in which each pixel cell includes a photosensitive element PD, a transfer transistor T1, a reset transistor T2, a source-follower (“SF”) transistor T3, and a select transistor T4.
During operation of Pa 110a, transfer transistor T1 receives transfer signal TXa, which transfers charge accumulated in PD to floating diffusion node FD. T2 is coupled between reset power supply RVDDa and FD to reset the pixel (e.g., to discharge or charge FD and/or PD to a preset voltage) under control of reset signal RSTa. FD is also coupled to control the gate of T3. T3 is coupled between source follower power supply SVDD and T4. T3 operates as a source-follower providing a high impedance connection to FD. Under control of select signal SELa, T4 selectively provides an output of pixel cell Pa 110a to readout column bit line BL. Similar operation of pixel cell 110b is achieved with a corresponding transfer signal TXb, reset power supply RVDDb, reset signal RSTb and select signal SELb.
In pixel cell 110a, PD and FD are reset by temporarily asserting reset signal RSTa and transfer signal TXa. An image accumulation window (exposure period) is commenced by de-asserting transfer signal TXa and permitting incident light to charge PD. As photo-generated electrons accumulate on PD, its voltage decreases. The voltage or charge on PD is indicative of the intensity of the light incident on PD during the exposure period. At the end of the exposure period, reset signal RSTa is de-asserted to isolate FD and transfer signal TXa is asserted to allow an exchange of charge between PD and FD, and hence the gate of T3. The charge transfer causes the voltage of FD to change by an amount which is proportional to photogenerated electrons accumulated on PD during the exposure period. This second voltage biases T3 which, in combination with select signal SELa being asserted, drives a signal from T4 to the readout column line. Data is then readout from pixel cell Pa 110a onto readout column bit line BL as an analog signal.
Generally speaking, miniaturization in image sensors results in smaller photodiodes which generate smaller amounts of charge for smaller amounts of incident light, where signals of smaller voltage and/or current levels are in turn generated for representation of the captured image. Such smaller signals are more susceptible to various types of noise.
Pixel readout noise depends upon a frequency bandwidth of bit line BL. Current techniques to address pixel readout noise include providing a large sample hold capacitance in a correlated double sampling (CDS) circuit (not shown) which is coupled for a differential sampling of bit line BL. Larger sample hold capacitance results in narrower frequency bandwidth characteristics for signaling on bit line BL, which in turn reduces signal noise. However, implementing such a larger sample hold capacitance requires image sensor circuitry to have a larger die size.
Another solution for reducing pixel readout noise is to provide a large gate size for source follower transistor T3. A larger SF gate size provides for better signal-to-noise characteristics, as compared to a smaller SF gate size. However, large SF gate size typically comes at the cost of photodiode size, which degrades sensitivity and full-well capacity of the photodiode. These and other current techniques to improve pixel noise generally have prohibitive trade-offs in terms of image sensor size.